Boat lift

ABSTRACT

One embodiment of a boat lift utilizes elongated, upendable floats of polyurethane material which project more deeply into the water when the floats are upright than when they are prone. Attachment of the carriage of the lift to the floats adjacent their &#39;&#39;&#39;&#39;sinking ends&#39;&#39;&#39;&#39; causes the carriage to be submerged when the floats are upright and to be raised for supporting a boat when the floats are in their prone positions. A second embodiment of the lift utilizes hollow pontoons which may be filled with water or air to cause submersion or flotation of the lift and which are rotatable between float positions in which air supplied to the pontoons is trapped therein, and sink positions in which air is allowed to exhaust from the pontoon while water flows thereinto. Diametrically opposed air pockets on the outside of the pontoons cause automatic return thereof from their sink positions to their float positions, and outlet tubes projecting downwardly from the pontoons when the latter are in their float positions prevent air from escaping from the pontoons during rough water conditions.

United States Patent 1 July 22, 1975 King 1 1 BOAT LIFT [76] Inventor:Arthur Shelley King, 8021 Cherokee Ln.. Leawood, Kans. 66206 [22] Filed:Jan. 9, 1974 [21] App]. No.: 431,855

Related US. Application Data Primary ExaminerTrygve M. Blix AssistantExaminerCharles E. Frankfort Attorney. Agent, or FirmSchmidt, Johnson.Hovey & Williams [57] ABSTRACT One embodiment of a boat lift utilizeselongated, upendable floats of polyurethane material which project moredeeply into the water when the floats are upright than when they areprone. Attachment of the carriage of the lift to the floats adjacenttheir sinking ends" causes the carriage to be submerged when the floatsare upright and to be raised for supporting a boat when the floats arein their prone positions. A second embodiment of the lift utilizeshollow pontoons which may be filled with water or air to causesubmersion or flotation of the lift and which are rotatable betweenfloat positions in which air supplied to the pontoons is trappedtherein, and sink positions in which air is allowed to exhaust from thepontoon while water flows thereinto. Diametrically opposed air pocketson the outside of the pontoons cause automatic return thereof from theirsink positions to their float positions, and outlet tubes projectingdownwardly from the pontoons when the latter are in their floatpositions prevent air from escaping from the pontoons during rough waterconditions.

11 Claims, 8 Drawing Figures PATENTEDJUL 22 ms SHEET N mb BOAT LIFTCROSS REFERENCES This is a continuation-in-part of my co-pendingapplication Ser. No. 326,536, filed'lan. 24, 1973, entitled BOAT LIFTand now abandoned.

This invention relates to an improved boat lift for use in storing aboat in an elevated position. above the water. so that the boat mayremain free of algae and similar collections about its hull when not inuse.

An important object of the present invention is to provide a boat liftwhich is not only extremely effective in raising boats of various sizesand displacements, but is also relatively inexpensive to fabricate andrelatively simple to operate.

Another important object of this invention is to provide a boat liftwhich eliminates the need for pneumatic equipment often associated withconventional lifts for pumping air or water into and out of flotationcompartments of the lift through the provision of one or more upendablefloats which serve to raise or lower the lift by virtue of thedifference in their flotation properties when the floats are upright ascompared with a prone position. To this end. advantage has been taken ofthe fact that the floats must displace the same amount of water ineither an upright or a prone position. but a selected point on one ofthe floats may change positions drastically from above the water to wellbelow the same when the float is shifted from one position to the other.Therefore. by connecting the floats to the boat carriage of the lift atsuch preselected points on the floats. the carriage will move inaccordance with its mounting points between submerged and elevatedpositions when the floats are operated between their upended and pronepositions.

An additional important object of the present invention is to provide aboat lift embodying the flotation concepts above set forth wherein allfloats of the lift may be operated in unison and by an individualpositioned within a boat overlying the lift.

A further important object of the instant invention is the provision ofa second type of boat lift. pneumatically operated. wherein pontoons ofthe lift are rotatable between float positions in which air is trappedwithin the pontoons to maintain the boat out of the water and sinkpositions in which the trapped air is exhausted from the pontoons andwater is allowed to fill the same for submerging the lift.

A still further important object of this invention is to providepneumatically controlled pontoons as aforesaid. having means forresisting the escape of trapped air from the pontoons even during roughwater conditions when the pontoons tend to rise and fall. leaving theundersides thereof periodically open to the atmosphere.

Another important object of this invention is the provision of means forautomatically returning the pneumatic pontoons from their sink positionsto their float positions.

An additional important object of the instant invention is the provisionof a pneumatic boat lift which may be submerged by an individual locatedwithin a boat supported by the lift.

In the drawings:

FIG. 1 is an elevational view showing one version of the lift of thepresent invention with a boat in position thereon. alternative positionsfor the upendable floats of the lift being indicated by solid lines andin broken lines respectively;

FIG. 2 is a top plan view of the lift;

FIG. 3 is a rear elevational view of the lift and boat;

FIG. 4 is an enlarged, fragmentary elevational view of one of the floatsshowing the pivotal axis of the float located at one end thereof ratherthan between its ends as in FIGS. l3;

FIG. 5 is a view similar to FIG. 1 ofa second version of the lift.alternative positions of the lift being indicated by solid lines andbroken lines respectively;

FIG. 6 is a rear elevational view of the lift and boat of FIG. 5;

FIG. 7 is an enlarged. fragmentary end view of one of the pontoonsillustrating its manner of operation; and

FIG. 8 is an enlarged, cross-sectional view of one of the floats takenalong line 88 of FIG. 5.

The lift 10 illustrated in FIGS. 1-4 is designed to accommodate a boat12 and has a main frame or carriage 14 provided with a pair offore-and-aft extending, laterally spacedapart sections 16 which areinterconnected by anumber of cross-pieces 18. Rings 20 or the like atthe forward ends of carriage sections 16 adapt the lift 10 for securanceto a dock or the like. and suitable roller and guiding structures 22, 24and 26 engage the hull of boat 12 when it is supported by carriage 14.

Four upendable. elongated floats 28, preferably blocks of polyurethanematerial. are swingably mounted on carriage 14 by virtue of a pair offore-andaft spaced. transversely extending torque members 30, each ofwhich is rotatably mounted on the sections 16 by suitable bearing means31. Each member 30 has a pair of mounting plates 32 at opposite endsthereof for rigid attachment to corresponding floats 28 at preselectedpoints along the lengths of the floats 28 for movement with the members30 when they are rotated. Each pair of the floats 28 interconnected bythe member 30 is provided with a crank 34 rigidly affixed to itscorresponding member 30 for applying torque thereto when crank 34 isactuated. A fore-and-aft extending link 36 operably interconnects thetwo cranks 34, and a second link 38 operably connects the rearmost crank34 with a hand lever 40 pivotally coupled with carriage 14, wherebycranks 34 may be operated in unison upon swinging of lever 40.Preferably, the lever 40 is so situated that a person disposed withinboat 12 may conveniently operate the same to actuate cranks 34. Aspring-loaded latch assembly 41, associated with hand lever 40,releasably holds lever 40 in either its forwardmost position illustratedin FIGS. 1 and 2, or its rearmost position shown in phantom in FIG. 1,thereby retaining the floats 28 in either their prone or uprightpositions.

As above set forth, the members 30 may be secured to the floats 28 atany of a number of selected positions along their lengths. the positionchosen being determined after due consideration to the size and weightof the boat which will be accommodated by lift 10. In this respect,FIGS. 1-3 illustrate the members 30 joined to their corresponding floats28 at points between the opposed ends of the floats 28, while FIG. 4illustrates an arrangement wherein a member 30 is secured to its float28 on one end of the latter. The point of attachment of members 30 totheir floats 28 is important inasmuch as such attachment points serve asthe axes of rotation for the floats 28, thereby positively affecting thedegree of flotation of the lift in the manner hereinafter made apparent.

Operation of the lift 10 is based upon the fact that the floats 28 willextend more deeply into the water when placed on end than when laid ontheir sides in prone positions. While it is, of course, understood thatthe displacement of the floats 28 is the same regardless of whether theylie in prone positions against the water or stand on end in uprightpositions, nonetheless. by careful selection of points adjacent thesinking ends of the floats 28 for attachment of the carriage 14, thelatter may be held out of the water as shown in FIGS. 1 and 3 when thefloats are in their prone positions, and held in a submerged conditionas shown by broken lines in FIG. 1 when the floats 28 are in theirupended positions. Therefore, it is but necessary to select a point ofattachment of carriage 14 to the floats 28 which is sufficient toaccommodate the weight of the boat intended to be used in connectionwith lift 10. It should be apparent that by selecting a connection pointon the very end of each float 28 as shown in FIG. 4, the carriage 14will be more deeply submerged and will rise slightly less out of thewater than when the carriage 14 is attached in the manner of FIGS. l-3.

When it is desired to remove the boat 12 from the water it is butnecessary to shift the hand lever 40 rearwardly from its FIG. 1 positionso that the cranks 34 operate in unison to swing floats 28 from theirprone positions to their upright positions. The carriage 14 is therebyfully submerged, and the upright floats 28 provide guides for centeringboat 12 is overlying relationship to the submerged carriage 14. The boatoperator need then only swing the hand lever 40 forwardly to lowerfloats 28 to their prone positions, hence raising carriage 14 andlifting boat 12 completely out of the water.

The boat 12 and lift 10 may be left in this condition for prolongedperiods with the assurance that the boat 12 will remain high and dry.

It has been found that an additional benefit is obtained by locating theattachment points of the carriage 14 to floats 28 closely adjacent or atthe sinking end of the floats 28. This advantage is noted duringswinging of the floats 28 from their upended positions to their pronepositions at which time the floats 28 tend to return themselves to theirprone positions once their respective centers of gravity have passed atrue vertical relationship with the pivot point of the floats 28. Inthis manner, the effort required to shift lever 40 is held to a minimum,hence facilitating the overall operation of lift 10 and making itpossible for individuals of all ages and physical characteristics to usethe same.

It is to be further noted that the floats 28, because of theirconvenient locations on opposite sides of carriage 14, may serve asconvenient walkways for persons entering or leaving boat 12 when thelatter is raised. In this respect, it is of course desirable that thefloats 28 be of sufficient size to locate their upper surfaces above thewaterline when floats 28 are in their prone positions. Moreover, arectangular configuration for the floats 28 provides a flat walkingsurface compared to other nonpolygonal configurations.

With reference now to FIGS. 5-8, a second embodiment of the boat lift istherein disclosed and denoted by the numeral 42 for supporting the boat12. The carriage 44 of lift 42 is substantially identical to carriage 14with the exception that carriage 44 is adapted for use not with thesolid upendable floats 28 of lift 10, but rather for use with pontoons46 which are responsive to the introduction of air or water thereinto tocontrol raising and lowering of the lift 42. To this end, the carriage44 has, along each side thereof, a pair of fore-andaft. spaced-apart,downwardly extending struts 48 having bearing units 50 at their lowerends for journaling a shaft 52 which projects from the proximal end of apontoon 46, thus rotatably supporting the pontoon 46.

EAch pontoon 46 is of elongated, generally tubular configuration havinghemispherical end closures 54 and a continuous peripheral wall 56 whichdefines an air or water chamber 58 within the body of pontoon 46. Therearmost of the end closures 54 of each pontoon 46 is completely closedat its junction with the shaft 52, but the forward closure 54 isprovided with an air inlet 60 therein disposed in concentricrelationship to the proximal shaft 52 and adapted for coupling with asupply line 62 associated with the bearing 50 which leads to an air pump(not shown). As illustrated, each pontoon 46 is mounted for rotationabout its longitudinal axis, such axis coinciding with that of theshafts 52, inlet 60 and bearings 50.

Each pontoon 46 is also provided with a pair of inletoutlet tubes'64 and66 which are secured to the outside of wall 56 in communication withchamber 58 for the passage of air and water into and out of chamber 58during floating and sinking operations of the lift 42. The tube 64 is oflarger capacity than tube 66 and projects radially from pontoon 46 forprotrusion downwardly into the water when the pontoon 46 is in theposition of FIGS. 5, 6 and 8. On the other hand, the tube 66 is slenderand is circumferentially spaced from tube 64 so that when pontoon 46 isrotated in the direction illustrated in FIG. 7, tube 66 leads tube 64and breaks the surface of the water before the pontoon 46 has beenrotated a full 90. Tube 66 extends generally in parallelism with tube 64for projection with the latter downwardly into the water when thepontoon 46 is in the position of FIGS. 5, 6 and 8.

A pair of air pockets 68 are disposed in diametrically opposed positionson each pontoon 46. The pockets 68 are located on the outside ofpontoons 46 and are sealed from the chambers 58 of the latter by thewalls 56 of the pontoons 46. As hereinafter explained, pockets 68 servethe purpose of returning the pontoons 46 to their normal float positionsfrom their sink positions approximately 45 beyond the rotative positionillustrated in FIG. 7.

Rotation of the pontoons 46 in unison is effected by a cable arrangementwhich includes a pair of side-byside, individually rotatable pulleys 70(FIG. 7) mounted beneath carriage 44, each of the pulleys 70 having acable 72 from a corresponding pontoon 46 entrained therearound. Anoff-center lug 74 on each pontoon 46 secures one end of thecorresponding cable 72 to the pontoon 46, and a conduit 76 receives thetwo cables 72 after entrainment around the pulleys 70 for guiding cables72 during actuation thereof and for 10- cating their opposite ends atconvenient location above carriage 44 for actuation by a person locatedwithin boat 12. Preferably, the actuatable ends of the cables 72 arejoined to a common gripping element (not shown) which not onlyfacilitates manipulation of cables 72, but also precludes accidentalrelease of the cable 72 through conduit 76.

A third pontoon 78 is located adjacent the front of carriage 44 andextends transversely across the latter to form the apex of an imaginarytriangle defined by the two rear pontoons 46 and the front pontoon 78.Pontoon 78 is similar in general construction to pontoons 46, but it isnot rotatable and'has a constant supply of air trapped therein forremaining in a continuously floating condition. Thus. as illustrated inFIG. 5, pontoon 78 serves as a pivot for the remainder of lift 42 duringoperation thereof. One or more rings 80 may be provided at theforwardmost end of carriage 44 for use in securing the lift 42 to a dockor like structure.

The operation of lift 42 may best be illustrated by assuming that thelift 42 is initially in its raised position supporting the boat 12 outof the water as illustrated by solid lines in FIG. 5 and also in FIG. 6.Normally, the forward end of the lift 42 will be loosely secured to adock through rings 80 to permit actuation of the lift 42 to the brokenline position of FIG. 5.

When the lift 42 is in its raised condition, the pontoons 46 thereof aredisposed with their inlet-outlet tubes 64 and 66 pointed downwardly intothe body of water. Therefore, air that is pumped into the chamber 58 ofeach pontoon 46 by its supply line 62, is trapped by the water whichsurrounds and seeks to enter tubes 64 and 66. So long as pressure ofsufficient magnitude is supplied through line 62, air within eachchamber 58 cannot escape therefrom back through inlet 60 and,accordingly, the lift 42 floats.

To operate lift 42, it is but necessary for a person situated within theboat 12 to grasp the cables 72 conveniently located at the proper heightby conduit 76 anad exert a sufficient pull to rotate the pontoons 46 inopposite directions about their longitudinal axes beyond theintermediate rotative position of FIG. 7 until just less than a full 90rotation has been effected. In this respect, suitable stop means (notshown) could be provided to prevent a full 90 rotation and beyond sothat the pockets 68 of each pontoon 46 are never in a true verticalrelationship with one another.

Movement of the pontoons 46 in this manner exposes the slender tubes 66thereof to the atmosphere and allows the air previously trapped thereinto escape while water floods the chambers 58 through tubes 64.Initially. because of the location of each tube 64 on its pontoon 46,both air and water simultaneously pass through the tube 64, the waterflowing into chamber 58 and the air rushing outwardly therefrom.However, it will be appreciated that this action is brief and that airis exhausted primarily through each tube 66 and water is receivedprimarily through each tube 64.

Flooding of the chambers 58 of pontoons 46 causes the lift 42 togradually submerge to its broken line position of FIG. 5 as the carriage44 pivots about the front pontoon 78. Thus. as soon as carriage 44 issubmerged sufficiently. the boat 12 is released by lift 42 and allowedto float on its own.

Once the pontoons 46 have become sufficiently flooded to lower the lift42, the operator may release cables 72 and the lift 42 will continuesinking until reaching its fully lowered position. Such release ofcables 72 causes the pontoons 46 to be automatically returned to theirfloat positions in which the tubes 64 and 66 project downwardly byvirtue of the return pockets 68 located on opposite sides of eachpontoon 46. It will be appreciated that because of the existence of thepockets 68, wwhich are always provided with a supply of trapped air, thepontoons 46 tend to seek an orientation in which both pockets 68 of eachpontoon 46 are at the same level. Thus. (using the pontoon 46 of FIG. 7as an example) with one pocket 68 in an approximately 7 oclock positionand the other in an approximately 1 Oclock position when a pontoon 46 isin its sink position to exhaust air and receive water, the pockets 68force their pontoon 46 to rotate clockwise when cable 72 is slackeneduntil equalization is once again established. Because the pocket 68 at 7oclock on pontoon 46 is submerged more deeply than the opposing pocket68 and yet is the same density and volume, a buoyancy force differentialis created that causes the pockets 68 to seek an equalized condition.

And because the lower pocket 68 is not in a true vertical relationshipwith the opposing pocket 68, its buoyancy force is not divided intoequal and oppositely projecting forces from a vertical line that wouldresult in a static condition. Moreover, it will be appreciated that thepresence of pockets 68 stabilizes the pontoons 46 when they are in theirfloat positions, such stabilization being especially important when theboat 12 is supported out of the water so that pontoons 46 are preventedfrom accidentally rotating during rough water to flood their chambers 58and submerge the lift 42.

The projection of tubes 64 and 66 of each pontoon 46 for a substantialdistance beyond the wall 56 thereof is important inasmuch as the outerends of the tubes 64 and 66 are thus not exposed to the atmosphere. evenin rough water conditions. In this manner, the pontoons .46 may thus bebounced completely out of water for short distances to expose portionsof the undersides thereof without water entering chambers 58 toaccidentally sink the lift 42 and hence float boat 12.

When it is desired to remove the boat 12 from the water it is butnecessary to sink the lift 42 in the manner above described and bringthe boat into overlying relationship with the submerged carriage 44.

Air is then pumped into the pontoons 46 to exhaust the water fromchambers 58 through tubes 64 and 66 until such time as a flotationcondition is reached, thereupon the lift 42 will swing upwardly toengage the hull of boat 12 and raise the same completely out of thewater. With the pontoons 46 in their float positions during exhaustionof the water from chambers 58, it is assured that the air pumped intopontoons 46 will remain trapped therein when the lift 42 has reached itsfully raised condition.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

l. A boat lift comprising:

a carriage;

a tubular, essentially closed, elongated pontoon having a continuousouter wall defining a chamber within the pontoon and provided with inletmeans and outlet means; and

means rotatably mounting said pontoon on the carriage for rotation ofthe pontoon about its longitudinal axis between a float position whereinsaid outlet means is submerged for trapping air within the chambersupplied through said inlet means and a sink position wherein saidoutlet means is open to the atmosphere for exhausting air from thechamber for entry of water into the latter.

2. A boat lift as claimed in claim I, wherein said outlet means includesa tube projecting outwardly from said wall for protrusion downwardlyinto the water when the pontoon is in its float position.

3. A boat lift as claimed in claim 2, wherein said tube both exhaustsair and receives water when the pontoon is initially rotated to its sinkposition.

4. A boat lift as claimed in claim 1, wherein said outlet means includesa pair of tubes projecting outwardly from said wall. one for primarilyreceiving water and the other for primarily discharging air when thepontoon is rotated to its sink position.

5. A boat lift comprising:

a carriage;

a tubular, essentially closed pontoon having a continuous outer walldefining a chamber within the pontoon and provided with inlet means andoutlet means; and

I means mounting said pontoon on the carriage for movement of thepontoon between a float position wherein said outlet means is submergedfor trapping air within the chamber supplied through said inlet meansand a sink position wherein said outlet means is open to the atmospherefor exhausting air from the chamber for entry of water into the latter,

said outlet means including a pair of tubes projecting outwardly fromsaid wall. one for primarily receiving water and the other for primarilydischarging air when the pontoon is moved to its sink position,

said pontoon being mounted on said carriage for rotation between saidpositions, said other tube being spaced from said one tube along thepath of rotation of the pontoon to project out of the water when thepontoon is rotated to its sink position,

6. A boat lift comprising:

a carriage;

a tubular, essentially closed. elongated pontoon having a continuousouter wall defining a chamber within the pontoon and provided with inletmeans and outlet means; and

means rotatably mounting said pontoon on the carriage for movement ofthe pontoon between a float position wherein said outlet means issubmerged for trapping air within the chamber supplied through saidinlet means and a sink position wherein said outlet means is open to theatmosphere for exhausting air from the chamber for entry of water intothe latter,

said pontoon being mounted for rotation about its longitudinal axisbetween said positions and being provided with means for automaticallyreturning the same from its sink position to its float position.

7. A boat lift as cliamed in claim 6, wherein said return meanscomprises a pair of diametrically opposed air pockets on the outside ofsaid pontoon sealed from said chamber 8. A boat lift comprising:

a carriage;

a tubular. essentially closed pontoon having a continuous outer walldefining a chamber withinthe pontoon and provided with inlet means andoutlet means; and

means mounting said pontoon on the carriage for movement of the pontoonbetween a float position wherein said outlet means is submerged fortrapping air within the chamber supplied through said inlet means and asink position wherein said outlet means is open to the atmosphere forexhausting air from the chamber for entry of water into the latter,

said pontoon being elongated and being mounted for rotation between saidpositions about its longitudinal axis, said outlet means being spacedlaterally from said axis of the pontoon.

9. A boat lift as claimed in claim 8, wherein said inlet is located atone end of said pontoon in association with said mounting means.

10. A boat lift claimed in claim I, wherein'said carriage is providedwith a second rotatably mounted pontoon spaced from said first-mentionedpontoon, said carriage further being provided with means for operatingthe pontoons simultaneously.

11. A boat lift as claimed in claim 10, wherein said operating meansincludes cable means operable from a boat overlying said carriage.

1. A boat lift comprising: a carriage; a tubular, essentially closed,elongated pontoon having a continuous outer wall defining a chamberwithin the pontoon and provided with inlet means and outlet means; andmeans rotatably mounting said pontoon on the carriage for rotation ofthe pontoon about its longitudinal axis between a float position whereinsaid outlet means is submerged for trapping air within the chambersupplied through said inlet means and a sink position wherein saidoutlet means is open to the atmosphere for exhausting air from thechamber for entry of water into the latter.
 2. A boat lift as claimed inclaim 1, wherein said outlet means includes a tube projecting outwardlyfrom said wall for protrusion downwardly into the water when the pontoonis in its float position.
 3. A boat lift as claimed in claim 2, whereinsaid tube both exhausts air and receives water when the pontoon isinitially rotated to its sink position.
 4. A boat lift as claimed inclaim 1, wherein said outlet means includes a pair of tubes projectingoutwardly from said wall, one for primarily receiving water and theother for primarily discharging air when the pontoon is rotated to itssink position.
 5. A boat lift comprising: a carriage; a tubular,essentially closed pontoon having a continuous outer wall defining achamber within the pontoon and provided with inlet means and outletmeans; and means mounting said pontoon on the carriage for movement ofthe pontoon between a float position wherein said outlet means issubmerged for trapping air within the chamber supplied through saidinlet means and a sink position wherein said outlet means is open to theatmosphere for exhausting air from the chamber for entry of water intothe latter, said outlet means including a pair of tubes projectingoutwardly from said wall, one for primarily receiving water and theother for primarily discharging air when the pontoon is moved to itssink position, said pontoon being mounted on said carriage for rotationbetween said positions, said other tube being spaced from said one tubealong the path of rotation of the pontoon to project out of the waterwhen the pontoon is rotated to its sink position.
 6. A boat liftcomprising: a carriage; a tubular, essentially closed, elongated pontoonhaving a contInuous outer wall defining a chamber within the pontoon andprovided with inlet means and outlet means; and means rotatably mountingsaid pontoon on the carriage for movement of the pontoon between a floatposition wherein said outlet means is submerged for trapping air withinthe chamber supplied through said inlet means and a sink positionwherein said outlet means is open to the atmosphere for exhausting airfrom the chamber for entry of water into the latter, said pontoon beingmounted for rotation about its longitudinal axis between said positionsand being provided with means for automatically returning the same fromits sink position to its float position.
 7. A boat lift as cliamed inclaim 6, wherein said return means comprises a pair of diametricallyopposed air pockets on the outside of said pontoon sealed from saidchamber.
 8. A boat lift comprising: a carriage; a tubular, essentiallyclosed pontoon having a continuous outer wall defining a chamber withinthe pontoon and provided with inlet means and outlet means; and meansmounting said pontoon on the carriage for movement of the pontoonbetween a float position wherein said outlet means is submerged fortrapping air within the chamber supplied through said inlet means and asink position wherein said outlet means is open to the atmosphere forexhausting air from the chamber for entry of water into the latter, saidpontoon being elongated and being mounted for rotation between saidpositions about its longitudinal axis, said outlet means being spacedlaterally from said axis of the pontoon.
 9. A boat lift as claimed inclaim 8, wherein said inlet is located at one end of said pontoon inassociation with said mounting means.
 10. A boat lift as claimed inclaim 1, wherein said carriage is provided with a second rotatablymounted pontoon spaced from said first-mentioned pontoon, said carriagefurther being provided with means for operating the pontoonssimultaneously.
 11. A boat lift as claimed in claim 10, wherein saidoperating means includes cable means operable from a boat overlying saidcarriage.